Current computed tomography systems scan an object to be examined with a conical beam originating from a focus and with a matrix-like detector array. A detector array supplies output data corresponding to a received radiation in the form of rays having a particular ray geometry.
The output data is filtered and then back-projected three-dimensionally in order to produce at least one slice of a layer of an object for examination.
Computed tomography methods of the indicated type are known under the term “filtered backprojection.” However, as 3D methods, that is to say in conjunction with a matrix-like detector array, known methods do not provide an image quality which is judged to be adequate in practice. This is because “cone beam artifacts” occur due to the use of the conical X-ray beam.
Additionally disadvantageous in these methods is that redundant data are not used, such as data produced during spiral scanning with a small table advance as a result of multiple irradiation of one and the same voxel, which results in the incomplete use of an imaging radiation dose administered to the object.
Furthermore, there are considerations in connection with 2D methods for image reconstruction to proceed in such a way that preliminary images in large numbers are calculated from output data by “filtered backprojection”, originating from sections of the focal path which are intrinsically inadequate for image reconstruction, the preliminary images being reformatted to form a final slice only in a second step. These 2D methods are less useful for detector arrays with a large width. That is, in the direction of the system axis, since then an extremely large number of preliminary images have to be processed, which is a problem even when there is a large amount of computing power available.